The prevalence of obesity (body mass index [BMI] e30 kg/m2) in US adults has skyrocketed over the past 30 years, and is currently as high or higher in veterans as it is in non-veterans. Thus, identifying effective strategies for treating obesity is both a public health and a VA priority. A variety of diet approaches have proven successful in achieving moderate weight loss in many individuals. Yet, most diet interventions fail to achieve meaningful weight loss in more than a few individuals. This failure likely results from inadequate adherence to the diet. It is widely felt, but not empirically shown, that targeting the diet to an individual's food preferences will enhance adherence, thereby improving weight loss outcomes. This study will test the commonly proposed assumption that helping patients choose a diet based on their dietary preferences will increase weight loss success relative to assigning or recommending one diet. The proposed study is a 2-arm randomized controlled trial involving 216 outpatients from Durham VA Medical Center. Participants must be obese (BMI e30 kg/m) VAMC outpatients without unstable health issues. Participants in the experimental arm (Choice) will select from two of the most widely studied diets for weight loss, either a low-carbohydrate, calorie-unrestricted diet (LCD) or a low- fat, reduced-calorie diet (LFD). This choice will be informed by results from a validated food preference questionnaire and a discussion of available diet options with trained personnel. As may occur in the clinical setting, the Choice participants will also have the opportunity to switch to the other diet after 3 months if unsuccessful or dissatisfied with their primary selection. The Choice intervention is designed to enhance the three psychological needs of a person according to self-determination theory (SDT): competence, relatedness, and, in particular, autonomy. This should maximize intrinsic motivation, thereby improving adherence to diet recommendations and increasing weight loss. Participants in the Control arm will be randomly assigned to follow one of the two diets for the duration of follow-up. All participants will receive diet-appropriate counseling in small group meetings every 2 weeks for 24 weeks, then monthly for another 24 weeks. All participants will also receive brief telephone counseling involving individual goal setting and problem solving halfway through each month in the latter 24 weeks. The primary outcome is weight change from baseline to 48 weeks. Secondary outcomes include adherence to diet by food frequency questionnaire and obesity-specific health-related quality of life. Exploratory analyses will examine whether the impact of choice versus lack of choice on weight loss is moderated by individual differences in the SDT constructs of autonomy orientation, competence, and relatedness. If assisting patients to choose their diet enhances adherence and increases weight loss, the results will support the provision of diet options to veterans and non-veterans alike, and bring us one step closer to remediating the obesity epidemic faced by the VA and other healthcare systems.